Airborne Auxiliary Power Unit, abbreviated as APU, is a small turbine engine mounted on the tail of an aircraft. Its main function is to supply power and gas sources, with a few APUs capable of providing additive thrust to the aircraft. Specifically, before taking off from the ground, an aircraft may do not need to rely on ground power and gas source vehicles to start the aircraft as its main engine may be started via power supply from the APU. While on the ground, the APU also supplies power and compressed air to ensure lighting and air-conditioning in the cabin and cockpit. During take-off of an aircraft, the APU can serve as a backup power source. After the aircraft is landed, lighting and air-conditioning of the aircraft are still powered by the APU. The functions of APU determine that its stability directly affects flight cost and quality of service of the aircraft.
Ignition of APU is realized via a starter. The structure and cyclic process of the aircraft gas turbine engine decides that it cannot be ignited autonomously. This is because that, if fuel injection and ignition is performed directly in a static engine, the air compressor does not rotate so as to provide pressure to the air and the gas could not move backward to rotate the turbine, in which way the combustion chamber and turbine guide vane will be burnt out. Therefore, the feature for starting the gas turbine engine is: the air flows at first, and then ignition occurs, namely, the engine needs to be rotated firstly and then be started. According to the above feature, before ignition, the engine must be rotated via the starter by consuming electric power.
The starting of an APU is the process of accelerating the APU rotator from a static state to a stable working state, namely, the working process for the APU rotator to accelerate its rotation speed from 0 to over 95%. During this process, whether the APU rotator may reach a required working rotation speed within a prescribed time and enters into a stable working state mainly depends on the torque the APU rotator obtains during the starting process. As the service time of the starter increases, its efficiency reduces gradually due to the decrease of contact of the carbon brush or increase of internal friction caused by internal field deformation, increasing loss of copper and iron, mechanical wear and the like, and the output power reduces accordingly. When the output power of the starter reduces to a certain degree, the starter cannot provide the APU rotator with an adequate torque, namely failure of the starter occurs.
APU starter is an important component of APU. Once the starter fails, it will directly cause the APU unable to start, and thus cause the aircraft unable to fly. According to statistics, failure of starter occupies nearly half of the total amount malfunctions of APU, and is a main cause affecting normal operation of APU. It is also a main problem that needs to be solved so as to improve maintenance of APU. Currently, there is not any effective way to maintain the APU starter except for breakdown maintenance. In addition, since performance of the starter deteriorates at a rapid speed (generally speaking, the time for a starter from entering into decline phase to the occurrence of malfunction is basically within 30 hours of flight), there is a need to react to the deterioration of performance of the APU starter rapidly so as to save time to prepare a standby component, which is very important for ensuring on-schedule operation of an aircraft. Meanwhile, it will help to control inventory more accurately, or even realize zero inventory.